1. Field of the Invention
The present invention relates to an exhaust gas purifier for an internal combustion engine.
2. Description of the Background Art
An exhaust gas purifier is formed to have an additive supplying unit (such as a fuel addition valve for injecting fuel to the exhaust gas) for supplying an additive to the exhaust gas, a catalyst device promoting oxidation of the additive supplied to the exhaust gas, and an exhaust filter arranged downstream of the catalyst for trapping particulate matter (PM) in the exhaust gas (see Japanese Patent Laying-Open No. 2004-143988).
In such an exhaust gas purifier, when the deposition of PM on the exhaust filter is assumed to have reached the limit, a PM regeneration control for burning the trapped PM is performed. In the PM regeneration control, the additive is supplied to the exhaust gas in order to increase the temperature of exhaust filter to a target value.
Consequently, the exhaust gas is heated to a high temperature by heat generation caused by oxidation of the additive in the catalyst device, and by the hot exhaust gas flowing to the exhaust filter, PM is burned. Further, PM on the filter is also burned by the heat resulting from oxidation of the additive at the exhaust filter.
In the exhaust gas purifier, catalytic function of the catalyst device may sometimes be deteriorated. When PM regeneration control takes place where catalytic function has been deteriorating, though the additive is supplied to the exhaust gas, oxidation of the additive at the catalyst device is insufficient, and therefore, what is supplied to the exhaust filter would be exhaust gas of low temperature (exhaust gas at a lower temperature than the exhaust gas supplied to the exhaust filter through the PM regeneration control with normally functioning catalyst device). Further, the exhaust gas contains much additive that has not been burned in the catalyst device. In other words, exhaust gas of relatively low temperature containing much additive is supplied to the exhaust filter.
Therefore, when the catalytic function is deteriorated, the temperature of exhaust filter is mainly increased by the oxidation of additive at the filter. At this time, much heat generated by the oxidation of additive builds up at the downstream portion of the filter, while heat build-up is insufficient at the upstream portion of the filter, so that the PM purifying ability on the upstream portion of the filter decreases. In such a case, PM is not fully burned out but remains on the upstream portion of the filter, possibly resulting in clogging of the filter.
In the exhaust gas purifier described in Japanese Patent Laying-Open No. 2004-143988, though it is possible to detect deterioration of catalytic function, no measure is taken for the decreased PM purifying ability as described above, and therefore, it is difficult to avoid clogging of the exhaust filter or the like.